I have been using CC.NET for a while and decided to try Team City.
The initial and default configuration is very easy, but how do I swap configuration files after code is checked out and before unit tests are run.

I am using TFS, NUnit.
1. When working locally I have configuration file pointing to local server.
2. On the build server TeamCity get's notification that I have checked-in code and builds new version.
3. Server runs unit tests

When on 3rd step server runs unit tests I need to swap configuration files that are pointing to other servers, not the ones I am using locally.